Chapter 13
The origin of species
Species are independently evolving lineages
• General lineage species concept: species are metapopulations that exchange
alleles frequently enough to comprise the same gene pool
Phenetic characters
• Make sure you can distinguish between defining a species by phenetic
characters and recognizing species according to phenetic characters.
• The distinction between "definition" and "recognition" is similar to the distinction
between the formal description of a species and the diagnosis of a species.
• Phenetic characters are all the observable or measureable characters of an
organism, including microscopic and physiological. Pretty much interchangeable
with “morphological”.
• The distinction is between features which are essential, in principle, and those
which are useful, in practice. It is a distinction between an explanation of what a
species ought to be and a description of what a species happens to be.
• formal definition is the description of a species, using phenetic characters
• i.e. describe the difference between a rose, gardenia and a magnolia
Some species concepts define species at a point in time; others define
species through evolutionary time
– Non-temporal species concepts
– Phenetic species (definition based on clustering of physical
characters)
– Typological species (definition based on similarity to a type specimen
or ideal type)
– Reproductive species (definition based on reproductive isolation)
– Ecological species (definition based on niche)
– Temporal species concept
– Cladistic species (definition based on segment of phylogenetic tree)
– Evolutionary species concept (definition based on vague "tendencies")
The phenetic species concept
– Phenetic species are defined as as populations distinguished by discrete clusters
of phenotypic characters.
– The phenetic species concept resembles the practical criteria commonly used to
describe species. A population can be defined as a species by a group of
phenetic characters sufficient to distinguish it from other populations.
Other ways to identify species
• Biological species concept: species are groups of interbreeding populations
that are reproductively isolated from other such groups
• Phylogenetic species concept: smallest possible group descending from a
common ancestor and recognizable by unique, derived traits
The biological species concept
– Ernst Mayr's Biological Species Concept (BSC) defines species as "groups of
interbreeding natural populations that are reproductively isolated from
other such groups" is the most widely accepted definition for species.
– Most discussion of speciation has been based on this species concept.
– That is, most hypotheses concern mechanisms which establish reproductive
isolation.
– In the BSC, species are unified by gene flow within a common gene pool;
gene flow involves migration and interbreeding.
Interbreeding between species is prevented by isolating mechanisms
– isolating mechanisms serve to prevent interbreeding and thereby establish
reproductive isolation of populations.
– Isolating mechanisms can be categorized as either premating (prezygotic) or
postmating (postzygotic).
Isolating barriers
• Premating isolating mechanisms prevent potential mates from two separate
populations from getting together.
• The most basic of premechanisms involve one or another variation on
allopatry (geographic separation, habitat separation, seasonal or temporal
separation).
• More complicated premating mechanisms involve behavior, biomechanics,
or gamete interaction to prevent gametes from coming together.
Premating mechanisms
• geographic separation
– populations are geographically separated
– (e.g. islands)
• habitat separation
– Populations occur in different habitats in the same general region
– (e.g. mangroves)
• seasonal or temporal separation
– Mating or flowering occur in different seasons
– Separation of photosynthesis and nitrogen fixing
Geographical variation
• A cline is a type of geographical variation
• Cline is the gradual change in the species’ phenotype and genotype frequencies
through a series of geographically separate populations (ex: ring species)
Postmating-prezygotic barriers
• Gametic incompatibility: sperm or pollen from one species fails to penetrate
and fertilize the egg of another species
• Postmating isolating mechanisms involve some form of genetic incompatibility,
such as reduced viability or reduced fertility of hybrid individuals.
• Haldane's rule notes that in species with genetic sex determination,
reduced viability or fertility is much more common in the heterogametic
sex (e.g., the sex that produces different gametes, the XY sex; in
mammals the heterogametic sex is male, in birds it is females).
• Postmating isolating mechanisms comprise a selection pressure that
favors premating mechanisms, to minimize the production of less-fit
offspring.
Postzygotic barriers
• Hybrids are produced but have low fitness
– Hybrid inviability
– Hybrid sterility
– Ecological inviability
– Behavioral sterility
Recognition species concept
– The recognition species concept places emphasis not on reproductive isolation
per se but on mechanisms for recognizing suitable mates.
– In this concept, a species is defined by mutual recognition of suitable mates
among the members of a population.
– Specific mate recognition system is the term given to a method of
recognizing mates that is shared by all the members of a population.
– In this concept, mate recognition is causal for the origin of species, while
reproductive isolation is a secondary consequence of mate recognition.
– The recognition species concept is thus associated with a more specific
hypothesis concerning speciation than is the more general biological species
concept.
Ecological species concept
– The ecological species concept places on emphasis on niche or adaptive zone
rather than reproductive isolation.
– This concept draws support from two phenomena.
– In the phenomenon called competitive exclusion, two or more species
cannot occupy the same niche. Either one will be better and beat out the
other one in competition, or (in the unlikely event that they are exactly
equal) chance drift in relative population size will eventually eliminate one
or the other.
– In the phenomenon called character displacement, similar species
which share a common resource will diverge from one another. This
effectively divides the shared niche and reduces competition.
Pluralistic species concept
– Whether or not a pluralistic species concept is the best choice is related to the
question of how many different ways there are for speciation to occur.
– If species can form by adaptive splitting within a population, and by specific
mate recognition systems, and by incidental reproductive isolation (i.e.,
allopatry), and by reorganization of genomic architecture, then a pluralistic
concept might be appropriate.
– But a pluralistic concept raises the question, is there anything essentially the
same in the various accounts? If so, shouldn't that "essence" define
species? And, if not, is the concept of species meaningful?
Causes of Speciation
• Geographic Isolation
o Scientists think that geographic isolation is a common way for the process
of speciation to begin: rivers change course, mountains rise, continents
drift, organisms migrate, and what was once a continuous population is
divided into two or more smaller populations.
o It doesn’t even need to be a physical barrier like a river that separates
two or more groups of organisms—it might just be unfavorable habitat
between the two populations that keeps them from mating with one
another.
• Reduction of Gene Flow
o However, speciation might also happen in a population with no specific
extrinsic barrier to gene flow. Imagine a situation in which a population
extends over a broad geographic range, and mating throughout the
population is not random. Individuals in the far west would have zero
chance of mating with individuals in the far eastern end of the range. So
we have reduced gene flow, but not total isolation.
Modes of Speciation
• The key to speciation is the evolution of genetic differences between the
incipient species.
• For a lineage to split once and for all, the two incipient species must have
genetic differences that are expressed in some way that causes matings between
them to either not happen or to be unsuccessful.
• These need not be huge genetic differences. A small change in the timing,
location, or rituals of mating could be enough. But still, some difference is
necessary. This change might evolve by natural selection or genetic drift.
Allopatric Speciation
• (allo = other, patric = place)
• Allopatric speciation is just a fancy name for speciation by geographic isolation,
discussed earlier.
• In this mode of speciation, something extrinsic to the organisms prevents two or
more groups from mating with each other regularly, eventually causing that
lineage to speciate. Isolation might occur because of great distance or a physical
barrier, such as a desert or river.
• Allopatric speciation can occur even if the barrier is a little “porous,” that is, even
if a few individuals can cross the barrier to mate with members of the other
group.
• In order for a speciation even to be considered “allopatric,” gene flow between
the soon-to-be species must be greatly reduced—but it doesn’t have to be
reduced completely to zero.
Peripatric Speciation
• Peripatric speciation is a special version of the allopatric speciation mode and
happens when one of the isolated populations has very few individuals.
• (peri = near, patric = place)
a small population isolated at the edge of a larger population
• Double disaster: Not only are the island fruit flies geographically isolated
from their mainland relatives, but only a few larvae have survived the
harrowing journey to end up colonizing the island.
• Rare genes survive: These few survivors just by chance carry some genes
that are rare in the mainland population. One of these rare genes happens
to cause a slight variation in the mating dance. Another causes a slight
difference in the shape of male genitalia. This is an example of the
founder effect.
• In peripatric speciation, small population size would make full-blown speciation a
more likely result of the geographic isolation because genetic drift acts more
quickly in small populations. Genetic drift, and perhaps strong selective
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pressures, would cause rapid genetic change in the small population. This
genetic change could lead to speciation.
The essential characteristic of this mode is that genetic drift plays a role in
speciation. There are likely many cases where a population is split into two
unequally-sized populations and they become separate species. However, it is
very difficult for us to tell after the fact what role genetic drift played in the
divergence of the two populations—so gathering evidence to support or refute
this mode is challenging.
Sympatric Speciation
• Unlike the previous modes, sympatric speciation does not require large-scale
geographic distance to reduce gene flow between parts of a population. How
could a randomly mating population reduce gene flow and speciate?
• (sym = same, patric = place)
• Merely exploiting a new niche may automatically reduce gene flow with
individuals exploiting the other niche. This may occasionally happen when, for
example, herbivorous insects try out a new host plant.
• For example, 200 years ago, the ancestors of apple maggot flies laid their eggs
only on hawthorns—but today, these flies lay eggs on hawthorns (which are
native to America) and domestic apples (which were introduced to America by
immigrants). Females generally choose to lay their eggs on the type of fruit they
grew up in, and males tend to look for mates on the type of fruit they grew up
in.
• So hawthorn flies generally end up mating with other hawthorn flies and apple
flies generally end up mating with other apple flies. This means that gene flow
between parts of the population that mate on different types of fruit is reduced.
This host shift from hawthorns to apples may be the first step toward sympatric
speciation—in fewer than 200 years, some genetic differences between these
two groups of flies have evolved.
Parapatric Speciation
• In parapatric speciation there is no specific extrinsic barrier to gene flow. The
population is continuous, but nonetheless, the population does not mate
randomly. Individuals are more likely to mate with their geographic neighbors
than with individuals in a different part of the population’s range. In this mode,
divergence may happen because of reduced gene flow within the population and
varying selection pressures across the population’s range.
• (para = beside, patric = place)
a continuously distributed population
• We may be observing the first steps of parapatric speciation in the grass species
Anthoxanthum odoratum
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Some of these plants live near mines where the soil has become contaminated
with heavy metals. The plants around the mines have experienced natural
selection for genotypes that are tolerant of heavy metals. Meanwhile,
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neighboring plants that don’t live in polluted soil have not undergone selection
for this trait.
The two types of plants are close enough that tolerant and non-tolerant
individuals could potentially fertilize each other—so they seem to meet the first
requirement of parapatric speciation, that of a continuous population. However,
the two types of plants have evolved different flowering times. This change could
be the first step in cutting off gene flow entirely between the two groups.
Although continuously distributed, different flowering times have begun to
reduce gene flow between metal-tolerant plants and metal-intolerant plants.
Parapatric speciation begins with the evolution of a hybrid zone
– The transition across which allele frequencies change may be broad or
narrow.
• A broad transition, extending across much of the total range, is
called cline.
• A narrow transition is called a hybrid zone or stepped cline.
– The width of a hybrid zone depends on hybrid fitness and on gene flow.
• The zone will be narrower with less migration (or lower rate of
gene flow) and/or with lower fitness for heterozygotes.
• The zone will be wider with more migration (higher rate of gene
flow) or with higher fitness for heterozygotes.
Evidence for Speciation
• In the summer of 1995, at least 15 iguanas survived Hurricane Marilyn on a raft
of uprooted trees. They rode the high seas for a month before colonizing the
Caribbean island, Anguilla. These few individuals were perhaps the first of their
species, Iguana iguana, to reach the island. If there were other intrepid Iguana
iguana colonizers of Anguilla, they died out before humans could record their
presence.
• Evolutionary biologists would love to know what happens next: will the colonizing
iguanas die out, will they survive and change only slightly, or will they become
reproductively isolated from other Iguana iguana and become a new species? We
could be watching the first steps of an allopatric speciation event, but in such a
short time we can’t be sure.
Bacterial species often defined by specific adaptations
Horizontal gene transfer makes classification difficult
Ultimately, species concepts are human artifacts
• Methods for recognizing species continue to improve
• Remember, the definition is arbitrary..it is done for our convenience…the
organisms couldn’t care!